Systematic error from unequal dropout between study groups.
Attrition bias occurs when participants who withdraw or are lost to follow-up differ systematically from those who remain, and this differs between comparison groups, distorting the apparent effect of an intervention. It is a particular concern in trials with long follow-up or burdensome interventions (e.g., rehab or exercise programmes), where dropout is often linked to poor tolerance, lack of benefit, or adverse outcomes. Intention-to-treat analysis and reporting of dropout rates/reasons by group are the usual ways to assess and mitigate it.
This guide was auto-drafted and is pending editorial review.